The invention relates to an electronic device for controlling and monitoring the electrical power supply to resistance welders, and is intended in particular for application in the field of resistance welding equipment equipped with rollers as used to join the overlapped longitudinal edges of tin cans and similar articles.
Conventionally, it is or crucial importance in this particular type of equipment to provide a system of supplying electrical power to the welding rollers in order to produce a weld exhibiting no breaks in continuity, devoid of spits, and of dimensions and quality remaining as constant as possible over time. In addition, the weld must conform to given specifications, for example, longitudinal and transverse dimensions and depth repeated identically in each single weld.
There are various power supply devices currently embraced by the art field in question, all aimed at achieving the objects mentioned above, in which the relevant electrical and electronic technology has been developed to greater or lesser degrees. One of the more significant solutions, leading in the first instance to higher welding speeds and by extension to the achievement of increased output in production, was that obtained by exploiting a sine wave type alternating current supplied at frequency increasing commensurately with the required welding speed; rotary or static frequency converters often appear in such systems. This type of device has several drawbacks however, including a tendency of the equipment to generate excessive heat, not least the power supply transformer and adjacent parts, which results in somewhat high levels of energy consumption and increased cooling requirements.
A second type of solution, designed to obviate the drawbacks mentioned above, uses alternating current not of sine waveshape but of an essentially square waveshape which affords undoubted advantages over the sine wave current: transmission of power to the welded material is more regular, and lower values of current amplitude and frequency can be adopted, thus considerably reducing the tendency of the equipment to overheat and saving on the energy consumed in welding, and in cooling the equipment.
By comparison, in effect, the square wave current rises swiftly to its crest value, and not after approximately one quarter of the period as in the case of the sine wave.
Nonetheless, the advantages of this solution are compromised inasmuch as the relative equipment is complex, and must also be oversized if the slope of the square wave welding current is to be maximized to best advantage.
If the slope is made sufficiently steep, in fact, inversion of the welding current will occur in a negligible interval of time, which in turn leads to a lower operating frequency.
This is indead one of the most important technical aims to be pursued in resistance welding, namely, the facility of operating with current of lower and lower frequency at undiminished levels of output, as it provides the solution to all those problems associated in particular with electric losses, for example eddy currents; such losses occur in direct proportion to the inductive phenomena underlying inductive reactances, which becomes increasingly high with higher welding frequencies.
Further notable advantages have been gained from a system that features a single static converter comprising a single rectifier and stabilizer module by which continuous voltage is supplied to a fully transistorized inverter supplying the power input to the welding transformer.
In accordance with the developments outlined thus far, the transformer will naturally incorporate structure capable of ensuring a suitably swift current inversion, and to minimizing electric losses.
The inverter is governed by an electronic regulator designed to compare the amplitude and frequency of the welding current with fixed reference values and thereupon to pilot the operation of the inverter in such a manner that the value of the output voltage will be as needed to produce the requisite current.
In this system, the waveform of the welding current is obtained by voltage regulation and dependent on proportioning of the transformer, and the selected amplitude value arrived at in the shortest interval possible commensurate with the selected transformer dimensions. Once the required amplitude is reached, the first pulse ceases and the current diminishes, according to a given time constant, to the point at which a further pulse is generated to recharge the circuit, and so on until the positive or negative state is reversed. Thus, the waveform produced in one half-period of the current exhibits a series of interconnected peaks, occurring at least around the crest value, which correspond to the weld spots.
The drawback most frequently occurring in this instance, however, is that the current peaks are not directly controlled and thus appear dissimilar to one from another; this reduces the quality of the weld, in which the spots are neither uniform in size and consistency nor distributed evenly along the lapped and joined edges.
Even with this last system, moreover, the welding current will always carry a continuous component, which gives rise to spits.